Apparatus for detecting flaws



06L 1951 w. c. BARNES ETAL 2,571,999

APPARATUS FOR DETECTING FLAWS Original Filed April 10, 1943 2 SHEETS-SHEET 1 /9 I K HUEDDUUEJI] HEDGE 4 1%32? 072$? 36 WQ/zef [fa/226$ m/ fi Weevil" Oct. 23, 1951 w. c. BARNES \ET'AL 2,571,999 I APPARATUS FOR DETECTING FLAWS Original Filed April 10, 1943 2 SHEETS-SHEET 2 PLATE CURRENT MA. who

TUBE CHARACTERISTIC- 5 W W? emu VOLTAGE vs.- PLATE MA. f

Patented Oct. 23, 1951 APPARATUS FOR DETECTING FLAWS Walter C. Barnes, Lake Bluff, and Henry W. Keevil, Evanston, Ill.

Continuation of application Serial No. 482,526, April 10, 1943. This application March 8, 1947,

Serial No. 733,310

16 Claims.

The testing of rails in railroad track by detector cars running thereon has now progressed to such a point that the chief obstacle to further improvement has been the difficulty of eliminating false indications due to physical characteris tics or irregularities resulting from wheel burns, gags caused by straightening the rail at the mill, magnetic spots, etc. when the detector is adjusted to detect small fissures. The residual magnetic system can easily be made sensitive enough to pick up extremely small fissures, but under such circumstances (as in any other system) there is an increased tendency to record irregularities other than fissures.

When any given apparatus records a large number of irregularities other than fissures, too much delay would be caused by stopping to make a hand check at each irregularity to determine by the reliable hand-check methods whether or not a fissure is present. It is, therefore, inevitable that under such conditions, the test crew must use its judgment. If, at the point where an irregularity has been detected, some surface defect is seen which would probably have caused the indication of an irregularity, the crew must often pass on without stopping for a hand check. In the past there has been no entirely reliable check on the judgment of the crew if no hand check was considered necessary.

In practice, some of the difiiculties have been avoided by the use of a coil running along the gauge side of the rail-head, but it has its handicaps and limitations.

According to the present invention, a high degree of distinguishing ability is provided with a pick-up which may run along the top of the rail. This is best accomplished through the cooperation of a new amplifying circuit and a new detector unit.

The detector unit comprises preferably a pair of coils with longitudinally disposed cores, each core having a pole approaching the rail in front of and to the rear of its coil and the cores having no air gap between them. Each core is relatively short, about one inch including the poles having been found to a very satisfactory length. One such coil can be used alone and has some advantage, at least in conjunction with the special amplifying circuit, in distinguishing between fissures and other irregularities, When two such 2 coils are used, however, particularly end to end, the distinguishing ability is further increased.

The amplifying circuit is preferably one which first cuts out all impulses below a predetermined voltage intensity and which then so amplifies and stretches out the very brief fissure impulses which are not cut oil as to acuate the recorder in spite of the brevity of the impulse.

Additional advantages and objects of the invention will be apparent from the following description and from the drawings in which Fig. 1 is an elevational view of a two-section detector car equipped for the practicing of the present invention;

Fig. 2 is a perspective view of the preferred form of pick-up unit of this invention as it ap* pears at the top of a rail-head;

Fig. 3 is a diagrammatic illustration indicating the difference in magnetic fields adjacent a fissure and adjacent non-fissure irregularities such as a wheel burn or a bolt hole;

Fig. 4 is a plan view of the pick-up unit shown in Fig. 2;

Figs. 5 and 6 are approximate oscillograms of the voltage induced in a single coil when said coil is passed across a fissure and a burn respectively;

Figs. 5A and 6A are theoretical oscillograms showing the voltage doubling to be expected if the two fissure voltage peaks are perfectly coincided and the much smaller voltage rise when coils with the same spacing pass over a burn-and no peaks are perfectly coincided;

Figs. 5B and 6B are views corresponding to Figs. 5A and 6A, but illustrate how this differential voltage increase could produce a higher combined voltage out of a pair of coils in the case of a fissure than is obtained by the same pair of coils in case of a burn even if with one coil the burn would give a higher voltage;

Figs. 7 and 8 are circuit diagrams for two amplifying circuits which may be used advantageously in the practice of this invention; and

Fig. 9 is a chart showing the tube characteristics of the first two tubes used in Fig. 7.

The invention has been illustrated in connection with its use in the residual magnetic system of testing rails for flaws, although in some of its aspects, it is useful in other systems such as the electro-inductive system. r

'cal.

In the electro-magnetic system, the rail is first energized by the magnets II, I2 and I3. Thereafter a pick-up um't I4 carried by a carriage I6 is moved along the rail beyond the effective field of the magnet I3 so as to be subject to the influence of only the residual fields surrounding the rail in the vicinity of irregularities therein. The magnets II, I2 and I3 are powerful electromagnets and their main purpose is to apply a strong magnetic flux longitudinally through the rail IT on which the detector cars I8 and I9 are running.- However, as the flux "enters the-rail, there is'atthe' rail surface a strong vertical flux passing from the magnet pole to the rail.

The effects of this vertical fiux may not be removed prior to exploring with the pick-up unit.

I4 and hence the pick-up unit I4 encounterstwo types of magnetic fields. The more common and more dangerous internal flaws such as the transverse fissure 2| are stronglyaffected by the; longitudinal flux in the rail-.and pr'oducera mag netic field which is generally of the type shown at 22 although no efiort has been madetoplot. the field exactly. Surface defects, however, are

relatively unaffected by the longitudinal flux since they are on a horizontal plane as indicated by'the-burn- 23, para'lel to the longitudinali flux; These defects, however; appear'to be traversedand strongly affected by the vertical fiux asit is enteringthe rail so that in effect suchhorizontal defectsare polarized with vertical poles. For example, the metal all along the burnv adjacent the'top of the rail mayretain' a vertical, magnetization in which case magnetic fiux. will tend to pass upward y from the burn and" out in. all directions to re-enter the railibeyondthe burn, as illustrated roughly at 24. Whether this is really the nature of the fieldor not .is not essential to a disclosure of this invention. In fact,. this inventionalso distinguishesbetween fissuresand bolt'holes, and the field of the latter is. believed to be more'like 22 except for bein spread out more andhaving a more gradual gradient as seen at 25.

An important object. of' this invention is to provide. av detecting apparatus which distinguishes so efiectively between these two types of fields that it may be given ahigh degree of sensitivity with respect to fissure. fields 22 so as to pick up and record even small fissures while being relatively insensitive to other irregularities or. to their fie1ds,.such as the field,24, so, as

. not to record these inspite of'its'high sensitivity for fissures. I

The pick-up unit'shown in,Figs. 2'and 4 has been found. to be" exceedingly efiective in distinguishing between. the fissure field 22' and the fields. such as 24 or, 25- of other irregularities. Thispick-up unit comprises a pair of coils which should be; connected in series opposition as indicated'in Fig; 7.. Each coil is wound on a core 28 (although thetermfcoil is also commonly used to. include a core 28). The cores may be identi- Each core has. twopoles, one in front of and one behindthecore, these poles thus being longitudinally spaced along the rail and prefpears that bestresults with a single coil are obt'ainedwitli a coil whose cores outside length is approximatelyone inch but under varying conditionsother lengths, especially shorter lengths,

may be best.

In Fig. 5, it will be seen that the fissure field 22 produces in a coil 26two narrow voltage peaks, a positive peak 3| and a negative peak 32. Other field irregularities on the other hand produce wider peaks 33 and 34 which are usually lower unless the fissure is quite small. With amplifying systems which have been used heretofore, especially when the detector did not bring out the differences of altitudes in the peaks 3i and 33 as effectively as the present pick-up unit, it has been difiicult to record some narrow peaks such. as 3I without also recording some wider peaks such' as the peak 33 even if they were slightly lower. The peak 33 may have more area under it than the peak 3|. Previous amplifiers have-been. more: responsive to a lower voltage, longer continued impulse than to an impulse of shorterduration but slightly higher voltage.

According to the present invention, amplifying circuits of-Figs. 7. and 8 are provided which are sufficiently quick. acting in response to voltage above a predetermined level to actuate the recording instruments under influence of a narrow hlghvoltage wave suchas SI, while quite sharply cuttin out any lower voltage wave such as 33.

These circuits are described hereinafter.

Because of'the fact that circuits havebeen too slow acting'to record thesnarrow peak 3Iunless considerably higher than necessary withaa wider peak, effort has been expended'heretoforein connection with electro-inductivesystems in trying to put two peaks together so as to give a Wave of longer duration. This may have been necessary to enable such systems to'pick up any but large fissures, but the longer duration of the fissure waves made them resemble mcrecloselythe burn waves 33 and hence did 'not helpin distinguishing between them.

The spacing preferred according to the present invention and its eifects arebestexplainedwith reference toFigs. 5A and 6A. InFig. 5A, the peaks SI. and. 32 represent the wave of the first coil 26, and the peaks I3'I (coinciding with 32) and I32 represent the corresponding wave of the second coil 21, the polarity of this Wavebeing reversed by the connection ofcoil 2l 'so that the first peak is negative. When the peak 32 of i one wave and' the peak I3I of the other'wave exactly coincide, they will produce a combined peak I35. Theoretically, this peak should be twice as high as the single peaks 32 and I3I although Figs. 5A and 6A are intended to illustrate-the principle involvedrather than necessarily-exactly what'happens.

In Fig. 6A, the peaks 33 and 34 represent the waves shown in Fig. 6 as :produced in the first coil 26, while the peaks I33 and I34 represent the corresponding wave produced in. the second coil 27. In this instance, it will be; observedthat the same spacing of thecoils 26 and-.21 does not produceexact coincidence ofthepeaks I 33 and 34; Accordingly, the combinedipeak' I36 will be considerably less than twiceas high asra single peak I33 or 34.

The spacingof the'coils-26'and 21 preferred in practicing the present invention is the spacing which perfectly coincides the fissure. peaks as illustrated in Fig. 5A,.or thexspacing which gives the maximum combined voltage I35, these'two statements meaning the same thing theoretically.

Both analysis and some preliminary test' data indicate that in the case of a single coilpassing over a fissure, the voltage'peaks'probablyoccur as the successive poles of the core pass over the fissure since these seem to be the times when there is the greatest rate of change of flux through the coil. In the case of 2 coils, this would mean that for perfect coinciding of the peaks, the first pole of the second coil should coincide in longitudinal Position with the second pole of the first coil. Longitudinal coincidence is approximated by having the cores in contact end to end. Longitudinal coincidence, or even this approximation of it would seem toproduce a low reluctance path from the first pole through both coils to the third pole and would seem to be objectionable because of the fact that any change in flux following this path would have no effect since the coils are connected in opposition. We have determined, however, that in spite of this theoretical analysis the two coils with their cores end to end are extremely effective in detecting fissures and also in distinguishing fissures from other irregularities. Apparently'the flux adjacent fissures isso concentrated that it greatly prefers the short path to the long path with the result that it flows first through one core and then through the other, and hence is decreasing in one core as it increases in the other.

Careful tests of the pair of coils illustrated used in conjunction with the ampliler of Fig. '7 indicates that any reduction of the spacing of the coils below half an inch is advantageous as compared to prior coil spacings and oscilloscope tests without the amplifier of Fig. '7 indicated that three-eighths of an inch is very desirable spacing. However, it is the response through an amplifier that counts, and, therefore, the zero spacing is preferred. The tests with the amplifier not only indicate that the coils are most responsive to fissures with zero spacing but also that with this same zero spacing, they are least responsive to other irregularities. In the case of the burn on which these tests were conducted, a spacing of one-eighth of an inch was found to increase the responsiveness to burns by almost ten per cent. With the corrugated rail on which these tests were made, a spacing of one-half an inch was found to increase the responsiveness to corrugations approximately twenty per cent. The comparison in each instance was with the zero spacing in which the coils were most responsive to fissures and least responsive to burns, and corrugations. This explains why, with these particular coils, the zero spacing is so greatly preferred in spite of the fact that any reduction of the spacing below one-half an inch represents an improvement over prior practice.

When, as with the illustrated structure, zero spacing is desirable, the two coils may be built separately and brought end to end or may be built as one structure. From some standpoints, this structure may be regarded as having a single central pole. In the illustrated form, this central pole has twice the cross-section of the end poles, but under some circumstances, it may be desirable to reduce the thickness at least to the thickness of the end poles. The maintenance of approximately the illustrated width transversely of the rail so that the coils will be responsive to the full width of the rail is at present preferred.

r -Of course, there may be some other explanation for getting the maximum combined voltage from fissures and best distinction between fissures and other irregularities when the coils are end to end. It seems clear that each coil would have someeffect on the other. In the first place, each coil would have an inductive effect on any current flow produced by the other coil. In the second place; each core would have some eifect'on the flow of magnetism through the other core. The first of these effects very probably creates a great differential between the external effects of a voltage induced in one coil and the external effect of a voltage induced in both coils simultaneously. If the effect of each core on the fiux in the other may cause a relatively sudden shifting of flux from one core to the other so that the flux is rapidly decreasing in one core while it is rapidly increasing in the other core, this would produce voltages of like polarity in the two coils since the coils are connected in series-opposition, so that a high combined voltage would be produced. If such is the underlying explanation, it appears that zero spacing produces the most sudden shift in flux from one core to the other in the case of fissures, but that this does not occur in the case of other irregularities.

Regardless of the underlying reason, it is an important observed fact that zero spacing produces a great differential between the maximum combined voltages of fissures and non-fissure irregularities, and it is that fact which makes the use of two coils with this spacing highly desirable as compared to the use of a single coil even though the use of the single coil of the one-inch length is in itself highly desirable as compared to the use of prior art single coils or even as compared to the use of prior art paired coils with the wide spacing of the prior art.

If it were found for any particular stretch of track that some particular type of non-fissure irregularities gave trouble with the end to end relationship, or any other spacing which yields the maximum fissure voltage, a slightly different spacing would be desirable if by taking advantage of a peculiarity of a particularly troublesome type of irregularity, it would give greater difference of voltage between the fissure impulses and the impulses of that particular type irregularities.

The amplifying circuits capable of taking the fullest advantage of the great voltage difference produced by the pick-ups of this invention are illustrated in Figs. 7 and 8. For the most part, these circuits will be sufficiently understood from the circuit diagrams but certain special features should be pointed out and disclosed in detail. It will be understood that the values appearing beside the condensers are microfarads and those beside the resistances are ohms.

One of the considerations believed to be important is that the lead 35 from the detector coils 26 and 21 be connected to the grid ll of the first amplifying tube 40 by a coupling of low or substantially no inductance. Preferably, the lead 36 is connected directly to the grid 4| as in the illustrated forms. An iron core transformer coupling might undesirably favor the longer impulses of surface defects while a condenser coupling might undesirably favor extraneous high frequency disturbances.

Another important characteristic of the cir-- cuits is the ability to cut off sharply any impulse below a predetermined voltage, preferably an adjustable voltage. In Fig. 7, this is accomplished mainly by the first two tubes in a manner which can best be explained by reference to Fig. 9 showing the tube characteristics.

The first tube 40 of the amplifier of Fig. 7 is a 6K7 tube with the grid 4| so connected that it normally operates at zero voltage, this resulting from the connection of the grid 4i through a grid leak 42 with common or ground wire 38 to which the cathodep43 is also directly connected.

With reference to the dotted line curve of vFig.

2&11159994 Quit is: seen that'the zero-grid voltage-pfor thistube-iseatpoint A. onthe curve, meaning that. the

flow-of; electronsthroug-h the tube from cathode;

43t051212tt8 Ad-isnearlyat the maximum'value possiblewith'the-plat'e voltage used. The knee above point A: is referred .to as thesaturationpoint. As. azresult, an impulse of positive polarity impressed on grid 4 I" by cells 26 and-21 will havepractically nd-eifect, its maximum efiect being anincrease ofplate current of only'approximately .1 milliam pere; Anegative impulse from coils 2'6 and 21' on thexother hand can have a very great-effect; ree d'ucing the. plate current from'approxi'matelyjits maximumtoca'pproximately zero. Thus the. tube is; practicallyunresponsive to. positive impulses butiis very sensitive toany negative impulses- It cuts ofii'pos'itive impulses andpasses negative; im.- pulses on to the next tube 50. The: grid. of this 'second. tube has-impressed on it a negative grid bias, the voltage of which is preferably ad'- iustab'l'e by means of a sliding potentiometercontact'sil;

- As -seen from the full line curveof Fig. 9, which shows I the characteristics of tube 50, a grid bias oi three and one h'alf volts negative puts theplate current between cathode 53' and plate 54 practically at zero. If the potentiometer contact39 is set for a'three and one-half volt negative bias, aismall" negative impulse from coils 26 and 21 amplifiedby tube lflwill impress a sufiicient positive voltage-ongrid 5 l 'to cause a platecurrent to flow to plate 5d; The voltage impulse resulting from= this plate current can be amplified by tubes 60; ll! and 86 to-operate a relay 9!. However, if

the-potentiometer contact 39 is set to apply a greater negative bias to grid 5i, a stronger negative impulse from coils 26 and2'l will be required to make'a plate current flow in tube 58 and ultimately cause operation of relay 9 I. Relay 9| will, of course, control the recording instruments, such as the pen on the moving chart and the'paint gun fo'rshooting a spot of painton the rail. Generally speaking, whatever bias is impressed on grid 5'l'inexcess of the three and one-half volt shut-oifbias, must be supplied by the amplified negative voltage of the coils 26 and 21 before the recording or actuating relay 9! will operate. H'ence, these first two tubes out 01f all'impulses of'lower voltage, thecut-ofi level being controlled by contact 39.

'The exact setting of the potentiometer contact 39 will depend on various considerations including the character of rail being tested. Where there are very few non-fissure irregularities, the grid setting may be such as to apply a bias to' grid 5| close to the shut-ofi'bias of three and one-half volts negativeso that fairly slight impulses from the pick-upcoils 26 and 2'! will produce operation of" the recorder relay 9i In this event, the occasional'severe non-fissure irregularities will also be recorded and will have to be eliminated by a hand check or by visual inspection, or by a rerun with a different setting of the potentiometer contact 39. Even a setting of 3 /2 volts, or perhaps 3 volts, would have at least some partial cutoff effect because operation on the knee of the curve represents inefiicient amplification; In track where there area large number of nonfissure irregularities, it may be desirable to set the potentiometer contact 39 for a negative bias of grid 5! considerably in excess of 3 /2 volts so that none or substantially noneofthe non-fissure lesszbetrecorded. Generallyspeaking; theseema y includel all except the very small fissuresv which are less important than the larger ones and, may be. caught at the nexttesting of the rail if' they grow-larger in the meantime. Burns can usually be: recognized from the moving car and a setting whichzrecords'many burns but few other non-' fissure-irregularities may sometimes bethe most practical setting.

Thetubes. 60 and 10 preferably have their grids 6i and ljso biased that the tubes operate at mid.- points B and C on their respective curves for most effective amplification of an impulse transmitted to them by the tube 50. The substantially straight portions of the curves of which B and C are the midpointsmay be referred to as the. normal op.- erating slope of the curve. The tube, 3.0 isJ-a powertube which operates the recorder relay 9.1;

It should be noted that one or more by-pass:

condensers 3'! 21111159 may be provided, the former beingacross the input leads and the latter between plate E lof tube 50 and the groundiconductor- 38. The purpose of this is to relatively reduce the voltage level of impulses resulting from sparkling, 60 cycle effects and'the like which have no' relationship. to fiaws in the rail and which are apparently produced independently of residual rail magnetism. Since the wave length of fissure impulses probably corresponds to a irequen'cy'no higher than about 35 cycles per second, the'condensers 3! and 59 are selectively moreeffective for the higher frequencies result ing from such external disturbances. Accordingly, substantially all of the external disturbances are eliminated by the condenser'59 so that very few, if any, false indications will be caused by them; Once the non-fissure impulses have been eliminated, the problem becomes one of actuating the recorder relay 9| with the fissure impulses in spite of their short duration and in spite of the fact-that some of them may be just barely strong enough to bepassed on by the first. two tubes lB-and150'. One important aidwhich can be .used if-desired for accomplishing this result seems to be in the provision of a grid leak 12 of relatively high resistance at the tube 10. The eiTect of this is to. make the impulses impressed on grid H by tube 60 through coupling condenser 68 last longer than wouldnormally be the case with .a conventionalresistance in the grid leak. In other words, the impulses are, relatively speaking, stretched. outv to have a longer duration. This gives thetube time enough to'operate the-relay .9l. This feature may be omitted, if desired, especially when it is not considered essential to record voltages just above the cut-off'value, as when the contact Sa is adjusted to out ofionly very'low voltages.

The circuit ofFig. 8 produces results similar to thoseof, Fig. '7, although in a somewhat different manner. Some features are quite similar. The. coils 26' and 2'! are directly connected to; grid 4.1" of tube-Ml. Aby-p-ass condenser 59 1870011? nected betweenplateb'l of 'tube'50 and ground wire 38. In this instance, the condenser willbe smaller than the condenser 59 since tubes 40" and 50; doynot amplify as much as tubes 40 and 59:. Avalue of .1 microiarad now seems proper. It should be noted, however, that under some circumstances, it may be desirable to eliminate thisby-pass fort-he sake of obtaining greater sensitivity. 3

Fig. 8} both the cut-off and stretch out functions are largelyaccomplishedby the same tube which is of the type known as a trigger tube.

plate current will flow to plate I65 and this in turn will impress a maximum charge on coupling condensers I69, thus enabling the tube 80 to actuate the relay 9|. It will be observed that the tube 160 not only greatly amplifies the momentary intensity of the impulse but also stretches it out in duration due to the vary high charge imposed on condenser I69 which isnot instantaneously nullified by grid leak 82, even though the grid leak 82 is of much lower resistance than the grid leak 12 of Fig. 7.

It is desirable to provide a sliding contact I61 for shunting out a part of the plate circuit resistance I68 so as to adjust the selectivity of the circuit in weeding out the very slight impulses in the plate current of tube 1 60 when this plate current is at or near its minimum value. It should be understood that the cut-off bias of grid l6! does not reduce the current quite to zero.

A further sensitivity adjustment may be made at I62.

In both Figs. 7 and 8 there is preferably provided a jack 93 for plugging in a sensitometer or other test equipment. Likewise, a milliammeter 94 is preferably provided in the out-put circuit.

At the present time, the circuit of Fig. 7 is preferred to that of Fig. 8 because it is an exceptionally stable circuit and because the single adjustment of the potentiometer contact 39 is all that is required and very satisfactorily selects the voltage level below which impulses are killed.

A special advantage flows from amplifying only one side of the impulse wave produced by the pick-up coils 26 and 21. Some surface defects produce a wave which is of the opposite polarity from the fissure wave. The result is that when two coils are properly connected to the amplifier in such a way that their coinciding or overlapping half waves are of the polarity which will be amplified (negative in Fig. 7), the corresponding overlapping half waves of the surfaces defects will be in the opposite polarity which is not amplified. Hence, there will not be present in the portion of the impulse which is amplified any additive effect from the two coils while there will be such additive effect in the case of fissures. .With the two coils connected in series-opposition as here, the impulse of a single coil is naturally of lower voltage than if it were not held down by the idle coil in series with it. All of these factors assist in distinguishing between fissures and surface defects. It should be noted, however, that even where the fissures and surface defects produce waves of the same polarity, the voltage increasing effect resulting from the proper coil spacing in the case of fissures is so much greater than in the case of non-fissure irregularities that surprising powers of distinguishing are obtained.

The positive portion of the fissure wave could be used and would have to be used if the first tube were chosen for cutting out the low voltages. It is preferred, however, to amplify the voltages from the pick-up unit before cutting out the lower voltages so that the borderline effects in the cut-off tube will be small as compared to the current would be available.

range of voltages impressed upon it. In Fig. 7, the second tube has been chosen as the cut-ofi tube and hence the negative part of the iissure wave applied to the first tube produces the necessary positive voltage at the grid of the second tube. No matter which tube is chosen to serve as the cut-off tube, it is preferred that each tube preceding it operate near one end or the other of its output curve to eliminate the possibility of its passing on a voltage of polarity opposite to that which it is intended to pass on.

Where the apparatus is to be used in conjunction with a magnetizing system in which the polarity can be reversed, means should also be provided for reversing the polarity of the connections between the pick-up unit (coils 26 and Z'f) and an amplifying circuit so that it will always be the combined fissure wave of intended polarity which will be amplified.

Apparatus embodying the coils herein illustrated connected to the amplifier illustrated in Fig. 7 has been found to give exceptional results. With this apparatus, it is possible (if the sensitivity adjustment at 39 is not to high) to test the rail to within two or three inches of the rail end, if not all the way to the rail end. This is apparently because the magnetic disturbances adjacent the joint bars and the bolt holes are of relatively long wave length so that the circuit of Fig. '7 is able to discard the impulses resulting therefrom even though it would at the same time respond to a fissure impulse.

It will also be observed that since the two coils are connected in series-opposition, any stray impulses of the nature which might result from the passage of a fluctuating current through the rail will be balanced out.' This renders the system appropriate for use in rails where alternating currents are used either for signalling or train propulsion and it also renders the equipment satisfactory for use as part of the electro-inductive system of testing. In the latter system the rail is energized by a current passing therethrough and the coils run along a portion of the rail through which the current is passing. A different spacing of the coils might be found to give better results with such a system, especially if transverse coils are used in place of the coils here illustrated, although the optimum spacing could readily be found in accordance with the principles above described. Although the detecting apparatus is suitable for use in such a system and would have advantages there over apparatus now used in such systems, its greatest advantage is attained in connection with the residual magnetic system.

This same balancing-out effect can be obtained by connecting the coils in parallel with such a polarity of their connections that a common flux change will produce a mutual short-circuiting effect in which the voltage induced in each coil tends to draw through that c-oil the current which the other coil tends to produce. Thus one lead from the amplifier would be connected to the positive end of one coil and the negative end of the other coil so that the net voltage impressed on the lead is zero. When the two coils are differentially acted upon, as by passing over a fissure, they would both produce voltage of the same polarity on this lead and hence a relatively heavy With such connections, or with any other balancing-out provision, a different coupling with the amplifier and even a different amplifier may be desired.

At the present time, however, it is preferred to "curve. setting provides-a grid bias greater than the cut- -ofi value for the tube. With 'a setting'onlyslighttroublesome non-fissure which cant be seen'and hence necessitate handnstigate =use the series-opposition Jeonnection with the illustrated amplifiers, particularly that oi Figrl.

The efiectiveness of this combination can perhaps be best seen withreference to Figs. b and 6b. These figures illustrate the situation when thefissure is so small that with one'core voltage peaks produced by fissures are not as high as those produced by a non -fissure irregularity.

Thus the peak '23! is considerably lower than the peak 233. Horizontal line "11 represents the l voltage level of peaks232 and 331 which coincide.

Horizontal line b indicates the voltage level of peaks 234 and 333 which-dont coincide.

coincide, but merely overlap, their combined voltage peak 336 willonly extend to level 0. This "differential is--equally advantageousif it "has the other explanations mentioned, that the fissure -field produces a sudden "shift-of flux from one core-to the other while the non-fissure field does not.

Now if contact -39 in Fig. '7 is-adjusted so that the-cut-off point is at level 0, the combined wave 336 will be cut out whilethe combined wave 335, although resulting from loweruncombined voltages, will be passed on from -tube 5D to tube El) and amplified to operate the recording apparatus.

Although Figs. 5b and 6b are notintended to be mathematically exact, they represent an important principle embodied in this invention. -No matter what the setting of the contact -39, this principle will be partially'utilized if the setting provides a grid bias on the cut-oil knee Dof the It will be fully utilized so long as this 1y exceeding this tube cut-ofl value, this invention will come close to providing 'themaximum flow-detecting ability. With asettingconsiderably beyond thetube cut-o'fi value, the invention will come close to providing the maximum ability to distinguish between fissures and all non-fissure irregularities.

Certain advantages can be attained by'using two detector-amplifier units each including a pair of coils similar to those shown in Fig. 2 and an amplifier similar to that shown in Fig. 7, thecontact 39 in one unit being-set fora negative grid 'bias'slightly exceeding thecut-off value of the tube and in the'other unit beingset foragreater "grid bias which 'will 'cut out voltages induced in "unit and will not record-any low-power non-fissure irregularities such as many gags and magnetic spots. It will ordinarily govern the train crew when there is no visible non-fissure irregularity that would have caused the response of this unit. In thus eliminating someof the-most irregularities those checkingit will produce a much cleaner record, speed up the testing and result in fewer errors of judgment. Even when there is'a visiblenon-fissure'irregularity which .justifies the crew in disregarding this unit, "the high cut-ofi unit will serve as a safeguard against missing any fissures except the very small fissures. The crew will know that when the high 'cut-oiiunitresponds,

other than "with its usuatrail-end indication,

there isia verystrong-probability that a fissure' is presentin addition to-anyvisibleirregularity. In this situation, therefore, =the-crew shouldalways stop 1 and-make a hand-check unless 'thepresence of a fissure can otherwisebe determined or negatived.

Oi course-with both units the exact adjustment may depend somewhat on track conditions. If there are very iew extreme burns,-the hig-h eut- --unit1maybe-adjusted for a somewhat-lower cut-off value than if. there are a lot of extreme burns, the adjustment in any event being such that the number of responsesmade when no fissureis present will not be so great as to unduly retard the test run.

It maybe helpful to describe more completely the coils that have given these excellent results although wide departures from these-details may be found harmless. The coils that have been tested mostthoroughly are eachof-85G0 turns-#4-2 enameled wire, wound on a core oflz'iaminations of transformer steel thecore having-an inside length '(betweenits poles) of ,4 and the pole width ofthe core being 1 ",-and the width'of the laminations within the coil being /2", and "the outside length of the'core being 1", as previous- -ly noted. A core-height (to top of core) of' "-is required-to make room for the winding. Preferably the coreshould be-of-low reluctance.

Although the coil -'-above described is 'very desirable with any amplifying circuit, it should be emphasized that best results are obtainedin-conjunction with the amplifying features or the present-invention. :It-has previously been proposed to distinguish between fissures and surface defects by differential penrelays, one actuated only by the greater outputs. However, results com- 1 parable to those-of the present invention have not previously been obtained This is probably in part because "thedifierence in wave shapes was overlookedand the low sensitivity relay had to be so extremely low insensitivity in order to avoid -beof their relativelysh ort wave lengths. prior art suggestions have 'completelylacked the ing actuated by the relatively long wave-length surface defect impulses that it was only actuatedby relatively large transverse fissuresin-view Also the concept of-cuttingout within the amplifier everything below a given "voltage no matter what its wave shape, and the concept of pure voltage-response unaffected-by questions of durations resulting from difierent '-wave shapes between the fissure and non-fissure impulses inducedin the 'coil by themagnetic fie'ld-of the rail.

"From the foregoing, it'is seenthat a method and apparatus for detecting hidden flaws in rail by a car running on the rail has been devised which is 'far 'superior'to 'prior systems particularly in'its abilityto distinguish between small 'fissures and relatively severe nonafissure irregularities, and in its ability to'record the fissures while the impulses'resulting therefrom are of very short -durationialthough maintaining a clean record when impulses of longer "duration but resulting 'from'sur'face defects are encountered.

up unit adapted to'be .relativelymovedalong the body being tested and including fa pair .of coils connected infseries oppositi'on on cores arranged in tandem within less thanfone"half inch, of each other longitudinally "of the rail "and adapted to have a voltage induced therein by passing through the magnetic field in the vicinity of a fiaw and normally a lesser voltage induced therein by passing through the magnetic field in the vicinity of non-flaw defect, an amplifier associated with the pick-up unit including a device having a control element, said amplifier being responsive to peak voltage variations above a predetermined level applied to the control element even though the duration of the peak is as short as that resulting from a transverse fissure, and completely non-responsive to voltages below said level, circuit means setting said level at or about the maximum level of voltage waves of like polarity caused by superficial rail defects, and indicating means operated by said amplifier in response to impulses produced by fissures.

2. Flaw detecting apparatus including a pickup unit adapted to be relatively moved along the body being tested and including a pair of coils connected in series-opposition on cores arranged in tandem within less than one half inch of each other longitudinally of the rail and longitudinally disposed, adapted to have a voltage induced therein by passing through the magnetic field in the vicinity of a fiaw and normally a lesser voltage induced therein by passing through the magnetic field in the vicinity of non-flaw defect, an amplifier associated with the pick-up unit including a device having a control element, said amplifier being responsive to peak voltage variations above a predetermined level applied to said control element even though the duration of the peak is as short as that resulting from a transverse fissure, and completely non-responsive to voltages below said level, circuit means setting said level at or about the maximum level of voltage waves of like polarity caused by superficial rail defects, and indicating means operated by said amplifier in response to impulses produced by fissures.

3. In apparatus for detecting fiaws in rail after the rail has been longitudinally magnetized to set up characteristic magnetic fields in the vicinity of rail defects, the combination of a pair of induction coils connected in opposition and of such length and spacing therebetween as. to cause fissure defects in rail to set up voltage waves, the center half waves of which are in substantial phase relation forming a prime wave, while corresponding wave portions of non-fissure defects are less substantially in phase relation forming a composite wave of normally smaller amplitude than said prime wave, and an amplifier for amplifying the output from said coils, said-amplifier having at least two stages of amplification, each of said stages including an electron discharge device having at least a cathode, an anode, and a control grid, the prime wave at one of said stages being impressed on the control grid as a positive impulse, the control grid of said one stage being biased with a voltage which is a predetermined amount below the lower end of the normal operating slope of the tube characteristic curve of the electron discharge device, said predetermined amount being substantially equal to the normal positive strength of said out-of-phase, non-fissure voltage waves at said grid, the prime wave at the other of said stages being impressed on the control grid of that stage as a negative impulse, the control grid of said other stage being biased at substantially the upper end of the normal operating slope of the tube characteristic curve of the electron discharge device whereby said stage will amplify substantially only negative impulses.

. 4. In apparatus for detecting flaws in rail after I the rail' has been longitudinally magnetized to set up characteristic magnetic fields in the vi-- cinity of rail defects, the combination of a pair of induction coils connected in opposition and of such length and spacing therebetween as to cause fissure defects in rail to set up voltage waves, the center half waves of which are in substantial phase relation forming a prime wave, while corresponding wave portions of non-fissure defects are less substantially in phase relation forming a composite wave of normally smaller amplitude than said prime wave, and an amplifier for amplifyingthe output from said coils, said amplifier having at least two stages of amplification, one of said stages including a sharp cut-ofi tube having at least a cathode, an anode, and a control grid, the prime Wave at said one stage being impressed on the control grid as a positive impulse, the control grid of said one stage being biased with a voltage which is a predetermined amount smaller than the negative cut-off voltage, said predetermined amount being substantially equal to the normal positive strength of said out-of-phase, non-fissure voltage waves at said grid, the other of said two stages including a tube having at least a cathode, an anode, and a control grid, the prime wave at said other stage being impressed on the control grid of that stage as a negative impulse, the control grid of said other stage being biased at substantially the saturation point of the tube whereby said stage will substantially only amplify negative impulses.

5. In a flaw detection apparatus of the type in which a rail is subjected to an externally applied magnetic force and then inductively searched for the magnetic fields characteristic of the presence of flaws, the combination of a pickup including a pair of induction coils connected in series opposition and with a longitudinally disposed core in each coil and with front, intermediate, and rear pole means associated with said cores whereby the pickup will cause fissure defects in rail to set up voltage waves in said coils, the center half waves of which are in substantial phase relation and form a prime wave, while corresponding wave portions and non-fissure defects are less substantially in phase relation forming a composite Wave of normally smaller amplitude than said prime wave, and an amplifier for amplifying the output from said coils, said amplifier having at least two stages of amplifications, each of said stages including an electron discharge device havin at least a cathode, an anode, and a control grid, the prime wave at one of said stages being impressed on the control grid as a positive impulse, the control grid of said one stage being biased with a voltage which is a predetermined amount below the lower end of the normal operating slope of the tube characteristic curve of the electron discharge device, said predetermined amount being substantially equal to the normal positive strength of said out-of-phase, non-fissure voltage waves at said grid, the prime wave at the other of said stages being impressed on the control grid of that stage as a negative impulse, the control grid of said other stage being biased at substantially the upper end of the normal operating slope of the tube characteristic curve of the electron discharge device whereby said stage will amplify substantially only negative impulses.

6. In flaw detection apparatus of the type in which a rail is subjected to on externally applied magnetic force and then inductively searched for the magnetic fields characteristic of the presence of flaws, the combination er aapick up including apair of induction coils connected in'series opposition andwithalongitudinally disposed core in each coil and with front, intermediate, and rear pole meansassociated with said cores whereby the pickup will cause fissuredefects in rail to set up voltage waves in saidcoils, the center half waves of which are in substantial phaserelation and form .a prime Wave, while cor responding wave portions of non-fissure defects 1 are less substantially in phase relation forming a composite wave of normally smaller amplitude than'said prime wave, and an amplifier for ame plifyingthe output from said coils, said amplifier having at-least two stages of amplification, one of said stagesincluding a sharp cut-offtube having at least a cathode, an anode, and a control grid, the prime wave at said onestage being impressed on the control grid as a positive impulse, the control grid of said one stage being biased with a voltage which is a predetermined amount smaller than the (negative cut-off voltage, said predetermined amount being substantially equal to the normal positivestrengthof said out-of-phase, non-fissure voltage waves at said grid, the other of said two stages including a tube havingatleast a cathode, an anode, and a control grid, the prime wave at said other stage being impressed on the control grid of that stage as -'a negative impulse, the control grid of said other stage beingbiasedat substantially the sat uration point of the tube whereby said stage will substantially only amplify negative impulses.

'7.--In progressive flaw detecting apparatus, a pickup including a pair of coils having cores therethrough with their poles longitudinally 'disposed with respect to one another, with pole means for opposite-ends of each core, the pole means for ea-chcoil closest to the other coil being positioned within half an inch of the other such pole means longitudinally of the apparatus, said coils being connected in series opposition, whereby :fissure defects in the r-ail'will set up voltage waves, the center half waves of which are in substantial phase relationship and form a prime wave, while corresponding wave portions of non-fissure defects are less substantially 'in phase relation forming a composite wave of normally smalleramplitude than said primewave,

and an amplifier for amplifying the output from said coils, said amplifier having at least two stages of amplification, each of said stages including .an electron discharge device having at least a cathodaanranode, and a control grid, the prime wave-at one of said stages being impressed on the control grid as a positive impulse, the controlrgrid of said one stage being biased with a voltage which is a predetermined amount below the lower end of the normal operating slope of the tube characteristic curve of the electron discharge devices, said predetermined amount-being substantially equal to the .normal positive strength of said out-of-phase, non-fissure voltage waves .at said grid, the prime wave at the other of said stages being impressed on the control grid of that stage as anegative impulse,-the control grid of said ,otherstage being biased at substantially the upper end of the normal operat ing-slope of the tube characteristic curve of the electron discharge device whereby said stage will amplify substantially only negative impulses.

8. In progressive flaw detecting apparatus, a pickup including a pair of coils having cores therethrough with their poles longitudinally dis- PQSed with respect to one another, with pole 16 means for opposite ends of 5511 -.c'ore', .the ale meansforeach coil closest to the other coil being positioned Within half an inch of .the other. such pole meanslongitudinally of the apparatus,.said coils being connected in series opposition, wherebyfissure defects in the raillwill set up voltage waves,.the center half wavesof which are in substantial phase relationship and form .a prime wave, while corresponding wave portions of anon-fissure defects are less substantially in phase relationforminga composite Waveof normally smaller amplitude than said prime wave, and an amplifier for amplifying the output from said coils, said amplifier having at least .two

stages of amplification, one .ofsaid stages 111 eluding a sharp cut-off tube having at least -.a cathode, an anode, and a control grid, the prime wave at saidone stage being impressed on the control grid as a positive impulse, the control'grid of said one stage being biased with .a voltage Which is a predetermined amount smaller than thenegative cut-off voltage, said predetermined amount being substantially equal to the normal positive strength of said out-of-phase, nonfissure voltage waves at said grid, the other of said two stagesincluding a tube having at least a cathode, an anode, and a control grid, the prime wave at said other stage being impressed on the control grid of that stage asa negativeimpulse, the control grid of said other stage being biased at substantially the saturation point of the tube whereby said stage will substantially only amplify negative impulses.

9. Progressive magnetic flaw detection apparatus including a pickup characterized by the normal peak fissure signal Voltage of one polarity produced thereby being greater than the normal peak signal voltage of the same polarity from a non-fissure defect, and a cascade amplifier having at least threestages of amplification, a first of said stages having a tube including at least a cathode, an anode, and a controlgrid, said-tube being so biased and loaded that signals of said one polarity will be amplified while signals of the opposite polarity will be substantially cut oil, a second of said stages .having a second tube .including at least a cathode, an anode, and :a control grid, the grid of said second tube being biased a predetermined amount beyond the cut-ofLsai'd amount being substantially-equal to said normal peak signal voltage of said polarity asapplied-to thegrid of said second tube from a non-fissure defect whereby the normal fissure'signal will be transmitted iby .said second stage to the substantial exclusion of the normal non-fissure defect signal, the third stage .of amplificationlincluding a tube having acathode,.an anode, and-a control grid, said tube being so biased and loaded as to strengthen the signal passed-by said second stage.

10. In fiaw'detection apparatus of the typelin which a rail is subjected to an externally applied magnetic force and then inductively SQELlChBdIfOlithe magnetic fields characteristic of the presence of flaws, the combination of a pickup includingla pair of induction coils connected in series oppositionand with a longitudinally disposed core in each coil and with front, intermediate, and rear pole means associated with said cores whereby the pickup will'cause .fissure defects in rail. to induce voltage waves in said coils, the centerxhalf waves of which are in substantial phase relation and ;iorm a prime wave, while corresponding wavelportions of non-fissure defects are less substantiallyin phase relation forming a composite Wave of.:normallysmaller amplitude thanlsaid prime wave, a recorder, and an amplifier for amplifying the output 01' said coils and operating said recorder, said amplifier having at least two stages of amplification, each of said stages including anelectron discharge device having at least a cathode, an anode, and a control grid, the prime wave at one of said stages being impressed on the control grid as a positive impulse, theco'ntr'ol grid: of said one stage being biased with a voltage which is a predetermined amount below the lower end of the normal operating" slope of the tube characteristic curve of the electron discharge device,. said predetermined amount being substantially equal to the normal positive strength of said out-of-phase, non-fissure voltage waves at said grid, the electron discharge device of the other of said stages being so biased and loaded as to strengthen the signal from said one stage, said other stage including means for extending the impulse from said one stage.

11. In progressive flaw detection apparatus the combination of a pickup including a pair of induction coils connected in series opposition and with a longitudinally disposed core in each coil and of such length and spacing therebetween as to cause fissure defects in rail to set up voltage waves in said coils, the center half waves or" which are in substantial phase relation and form a prime wave, while corresponding wave portions of non-fissure defects are less substantially in phase relation forming a composite wave of normally smaller amplitude than said prime wave, and an amplifier for ampliiying the output from said coils, said amplifier including a device having a control element, which device will not transmit impulses unless the voltage wave applied to the control element is greater than the peak voltage strength of said composite out-ofphase non-fissure voltage wave.

12. In progressive fiaw detection apparatus, a pickup including a pair of coils having cores therethrough with their poles substantially longitudinally disposed with respect to one another, with pole means for opposite ends of each core,

the pole means for each coil closest to the other coil being positioned within one-half an inch of the other such pole means longitudinally of the apparatus, said coils being connected in series opposition, whereby fissure defects in the rail will set up voltage waves, the center half waves of which are in substantially phase relation and form a prime wave, while corresponding wave portions of non-fissure defects are less substantially in phase relation forming a composite wave of normally smaller amplitude than said prime wave, and an amplifier for amplifying the output from said coils, said amplifier including a device having a control element, which device will not transmit impulses unless the voltage wave applied to the control element is greater than the peak voltage strength of said composite out-oiphase non-fissure voltage wave.

13. In progressive fiaw detection apparatus, a pickup unit adapted to be relatively moved longitudinally along a body to be tested, having therein a pair of coils connected to the amplifier in series opposition and with a longitudinally disposed core in each coil and with said cores each having a pole adjacent to the body between the coils and substantially in contact with the other core, whereby the pickup will cause fissure defects in the body to set up voltage waves in said coils, the center half waves of which are in substantial phase relation and form a prime wave, while corresponding wave portions of non-fissure defects are less substantially in phase relation forming a composite wave of normally smaller amplitude than said prime wave, and an amplifier for ampliiying the output from said coils, said amplifier including a device having a control element, which device will not transmit impulses unless the voltage wave applied to the control element is greater than the peak voltage strength of said composite out-of-phase non-fissure voltage wave.

14. Flaw detecting apparatus including a picli up unit adapted to be relatively moved along the body being tested and including a pair of induction coils connected in opposition and of i} such length and spacing therebetween as to cause fissure defects in the rail to set up voltage waves, the center half waves of which are in substantial phase relation forming a prime wave, while corresponding wave portions of non-fissure defects are less substantially in phase relation forming a composite wave of normally smaller amplitude than said prime wave, an amplifier associated with the pick-up unit including a device having a control element, said amplifier being responsive to peak voltage variations applied to the control element above a predetermined level even though the duration of the peak is as short as that resulting from a transverse fissure, and completely non-responsive to voltages below said level, circuit means setting said level at or about the maximum level of voltage of said composite wave, and indicating means operated by said amplifier in response to voltages above said predetermined level, whereby operation of the indicating means by superficial rail defects is substantially eliminated.

l5. Flaw detecting apparatus including a pick-up unit adapted to be relatively moved along the body being tested and including a pair of induction coils connected in opposition and of such length and spacing therebetween as to cause fissure defects in the rail to set up voltage waves, the center half waves of which are in substantial phase relation forming a prime wave, while corresponding wave portions of non-fissure defects are less substantially in phase relation forming a composite wave of normally smaller amplitude than said prime wave, each of said induction coils having associated therewith a core with longitudinally spaced poles and with the extreme longitudinal length of each core equal to approximately one inch, an amplifier associated with the pickup unit including a device having a control element, said amplifier being responsive to peak voltage variations above a predetermined level applied to the control element even though the duration of the peak is as short as that resulting from a transverse fissure, and completely non-responsive to voltages below said level, circuit means setting said level at or about the maximum voltage amplitude of said composite waves, and indicating means operated by said amplifier in response to impulses having voltage components whose amplitude is above said predetermined level.

16. In apparatus for detecting flaws in rail after the rail has been longitudinally magnetized thereby to set up characteristic magnetic fields in the vicinity of rail defects, the combination of a pair of induction coils connected in opposition and of such length and spacing therebetween as to cause fissure defects in the rail to set up voltage waves, the center half waves of which are in substantial phase relation forming a prime wave, while corresponding wave portions of non- 19 fissure defects are less substantially in phase REFERENCES CPIEU I relation forming a composite Wave of normally The following references are'of record in the smaller amplitude than said prime wave, means file of this patent: for eliminating impulses having a voltage am- .9 plitude substantially not greater than those 5 UNITED STATES PATENTS found in said composite waves, and means for Number Name Date amplifying and extending substantially all re- 2,109,455 Barnes et a1. Mar. 1, 1938 maining impulses greater than said amplitude of 2,133,687 Drake Oct. 18, 1938 elimination to a degree sufficient to actuate 2,223,371 Keevil 1 Dec. 3. 19 recorder apparatus. 10 2,228,294 Wurzbach Jan. 14, 1941 v 2,265,136 Barnes et a1 Dec. 9, 1941 WALTER C. BARNES. 2,461,252 Barnes et a1 Feb. 8, 1949 HENRY W. KEEVIL. 2,461,253 Barnes et a1 Feb. 8, 1949 

